Histological insights into trait acquisition in non-mammalian synapsids

Abstract

The synapsid stem lineage is classically known to document a step-wise pattern of trait acquisition however, evidence of homoplasy in synapsids is common and increased scrutiny within synapsid clades and less readily apparent microanatomical characters may not follow classic step-wise patterns. This dissertation examines a broad taxonomic sampling of synapsid fossils with special attention to the histological details of their dentitions to test for patterns of trait acquisition. In the first chapter, we describe the peculiar dentition of tapinocephalids, an early herbivorous group of synapsids. Tapinocephalids were the first synapsids to acquire mammal-like specializations such as precise tooth-to-tooth occlusion and a periodontal ligament while retaining ancestral features like alternating and rapid tooth replacement and wavy prismless enamel. The combination of traits that make up the tapinocephalid dentition are not represented in modern amniotes and appear to have been specialized to herbivory. In the second chapter, I compare seasonal physiological patterns captured by the growth marks in the dentine of Lystrosaurus from South Africa and Antarctica. While South African Lystrosaurus dentine shows evidence of severe and punctuated physiological stress, Antarctic specimens reveal a more constant, but less extreme amount of stress. This may reflect different responses to seasonality where Lystrosaurus living in South Africa experienced intense periods of drought while those in Antarctica experienced prolonged periods of darkness due to its high latitudinal position. These differences, although significant, are relatively minor suggesting a level of flexibility in Lystrosaurus physiology that would be expected from endothermic metabolisms. In the third chapter, we test for taphonomic proxies for tooth attachment in the proportion of empty tooth sockets in fossil synapsid jaws. We surveyed non-occluding jaws of most major clades and families of synapsids for the proportion of teeth retained in the jaw and found tooth attachment and replacement poorly explained proportion of teeth missing from a fossil jaw while enamel and diet were better predictors. We suggest function of the teeth may be influencing taphonomic preservation. We also discuss the significance of a lack of reliable correlations between traits and patterns of trait evolution as it pertains to the notion of acquiring “mammalness”.